mailbox sensor

This implements a mailbox sensor which sends emails to you when the door of your mailbox will be opened. It uses magnets to fix the sensor to the mailbox, so it will work only with metal mailboxes. The magnets are not too strong. The sensor will move, if somebody forces stuff into your mailbox. This is intended behaviour.

In addition to the 3D printed parts you will need:
* 1x Metal mailbox
* 2x [Neodym magnets, 15x15x3mm](https://www.maqna.de/neodym-quadermagnet-15x15x3mm-vernickelt-grade-n45)
* 1x [SmartHome Tür-/Fensterkontakt optisch](https://www.smarthome.de/geraete/smarthome-tuer-fensterkontakt-optisch-weiss)
* 1x [Fritzbox](https://avm.de/produkte/fritzbox/)

The SmartHome Tür-/Fensterkontakt is mounted inside your mailbox and is registered as device in the Fritzbox. Any change the sensor detects will trigger an email sent by the Fritzbox to you,